A power transmission/transformation system includes a gas insulation transmitter/transformer apparatus (hereinafter, referred to as “gas insulation apparatus”) such as a switchgear (such as a gas circuit breaker or a gas insulation disconnector), a gas insulation transformer, and a gas insulation pipe. A vessel of a gas insulation apparatus is filled with an insulation gas. The insulation gas serves as an electric insulation medium for preventing discharge between the vessel of the gas insulation apparatus and the electrical circuit in the vessel and as a cooling medium for suppressing temperature rise due to electric current. In addition, in a switchgear, the insulation gas serves as an arc-extinguishing medium for extinguishing arc discharge occurring at the switching operation. Currently, as an insulation gas filled in a high-voltage/large capacity gas insulation apparatus, a sulfur hexafluoride gas (hereinafter, referred to as “SF6 gas”) is widely used.
SF6 gas is an inactive gas having considerably high stability and is harmless and noncombustible. Further, SF6 gas is significantly excellent in the abovementioned insulation performance and the arc-extinguishing performance. Therefore, SF6 gas is suitably used for a high voltage gas insulation apparatus, contributing to size reduction of the gas insulation apparatus. However, SF6 gas has a global warming effect 23,900 times greater than CO2 gas, so that the use of CO2 gas as the insulation gas is now proposed.
As described above, accidental discharge may occur between the vessel and the electrical circuit at the time of use of the gas insulation apparatus. Further, in the switchgear, arc discharge may occur at the switching operation. It is known that when such discharge occurs, the insulation gas is changed into plasma to cause dissociation of the insulation gas molecules.
Even when the dissociation occurs due to discharge in the case where SF6 gas is used as the insulation gas, the majority of the SF6 molecules are recombined due to its high stability. It should be noted that there may be a case where a few sulfur (S) ions and a few fluorine (F) ions generated by the dissociation of the SF6 molecules react with a small amount of water existing in the vessel to generate a very small amount of cracked gas such as HF or SOF2 gas. However, the influence of the reaction is not strong enough to reduce the insulation performance, arc-extinguishing performance, and electric conducting performance of the gas insulation apparatus.
In the case where CO2 gas is used as the insulation gas, some oxygen (O) ions generated by dissociation of CO2 molecules react with metal constituting the most part of the gas insulation apparatus to generate CO molecules as a cracked gas without involving recombination. Thus, the amount of CO2 gas filled in the vessel is gradually reduced, while the amount of CO gas as the cracked gas is increased. CO2 gas is lower than SF6 gas in terms of the ratio of recombination after dissociation due to discharge. Therefore, in the case where CO2 gas is used as the insulation gas, the insulation performance or arc-extinguishing performance of the gas insulation apparatus is reduced with the use of the gas insulation apparatus more easily than in the case where SF6 gas is used. This often requires maintenance of the gas insulation apparatus or replenishment of the insulation gas.